Modern electronic devices such as cellular telephones, cameras, and computers often use digital image sensors, such as high dynamic range image sensors. Image sensors may sometimes be referred to herein as imagers. High dynamic range imagers are designed to capture scenes with light ranges that exceed the typical dynamic range of an individual linear pixel or an analog-to-digital converter. The dynamic range of a pixel can be defined as the ratio of minimum luminance or brightness in an image, which causes the pixel to saturate, to the brightness in an image, which achieves a signal-to-noise ratio (SNR) equal to one. The dynamic range of a scene can be expressed as the ratio of its highest illumination level to its lowest illumination level.
Examples of techniques for capturing high dynamic range images include combining multiple exposures of varying exposure times, utilizing partial reset level techniques, and providing pixels with logarithmic or other non-linear responses. With a multiple exposure technique, an image sensor takes a first long exposure and then takes a second short exposure. The two exposures are then combined into a high dynamic range image. Because the two exposures are taken at different times, however, the fast moving objects within a scene cannot be captured at the same spatial location. This leads to pronounced motion artifact in a reconstructed image. In addition, conventional digital image sensors require excessive amounts of storage, thereby increasing the cost of such sensors.
As will be explained, the present invention provides improved digital sensors and image reconstruction methods for creating high dynamic range images by using varying exposure times for capturing an image.